A system and probe for indicating the level of material in a vessel as a function of material capacitance comprising a resonant circuit including a capacitance probe adapted to be disposed in a vessel so as to be responsive to variations in capacitance as a function of material level. An rf oscillator has an output coupled to the resonant circuit and to a phase detector for detecting variations in phase angle as a function of probe capacitance. Level detection circuitry is responsive to an output of the phase detector and to a reference signal indicative of a predetermined level of material for indicating material level as a function of a difference between capacitance at the probe and the reference signal. In the preferred embodiments of the invention disclosed, an automatic calibration circuit adjusts the resonance characteristics of the parallel resonant circuit of the reference signal indicative of a predetermined reference material level.

Capacitance-type material level indicator

A new microprocessor controlled digital multimeter offers significant advantages to the user. Automatic calibration and correction routines ensure long term stability and simplify maintenance; self diagnostics greatly help troubleshooting and even warn of impending failures; software implementation of logic design simplifies hardware, improves reliability and adds capabilities such as fast auto ranging and software I/O control; the ability to process data permits averaging, linearization and normalization of input measurements, minimum/maximum storage and limit detection.

Method of automatically calibrating a microprocessor controlled digital multimeter

A portable circuit and method for performing a time study and analysis of bodily ionic characteristics as esophageal pH includes a transducer 11 for measuring pH and generating a proportional analog signal and an interface 12 for converting this signal into a representative digital signal including offset and gain controls 23,24 for permitting correction for transducer 11 calibration drift during the study period. A processor 14 receives and stores all digital signals for rapid transmission to a data analysis after the study period is ended. Esophageal pH values are measured and stored for a plurality of pre-study and post-study known conditions.

Portable circuit and method for performing a time study and analysis of bodily ionic characteristics

A system for indicating the level of material in a vessel as a function of material capacitance comprising a resonant circuit including a capacitance probe adapted to be disposed in a vessel so as to be responsive to variations in capacitance as a function of material level. An rf oscillator has outputs coupled to the resonant circuit and to a phase detector for detecting variations in phase angle as a function of probe capacitance. Level detection circuitry is responsive to an output of the phase detector and to a reference signal indicative of a predetermined level of material for indicating material level as a function of a difference between capacitance at the probe and the reference signal. An automatic calibration circuit adjusts the resonance characteristics of the parallel resonant circuit. A remote module provides facility for initiating a system-test and a system-calibrate operation, either individually or sequentially.

A system for automatically calibrating a main digital to analog converter (DAC) includes first and second adjustment DAC's, an offset DAC, a difference amplifier, an analog to digital converter (ADC), a microprocessor, and an analog switch To calibrate the offset of the main DAC, the microprocessor causes "zeros" to be applied to the digital inputs of the main DAC and the offset DAC The output of the offset DAC is coupled to an input of the difference amplifier The microprocessor causes a ground voltage to be applied to the second input of the difference amplifier via the analog switch The output of the difference amplifier is inputted to the ADC, which produces a first word The first word is stored by the microprocessor The analog switch is activated to apply the analog output voltage of the main DAC to the second input of the difference amplifier The ADC produces a second word which is compared to the first word The microprocessor computes a first correction word which is transmitted to the first adjustment DAC The first adjustment DAC produces a corresponding analog signal which is applied to a first correction input of the main DAC, causing the analog output voltage to be adjusted toward the ground voltage To calibrate the linearity of the main DAC, the microprocessor causes the analog voltage corresponding to the fourth most significant bit to be converted to a third digital word by means of the ADC, and then causes the analog output voltage corresponding to the sum of the less significant bits to be converted to a fourth word by means of the ADC The difference between the third and fourth words is used by the microprocessor to compute a second correction code The second correction code is converted to a second analog correction signal by the second DAC, causing the third analog output voltage to differ from the second analog output voltage by an amount corresponding to one least significant bit

Method and apparatus for automatically calibrating a digital to analog converter

In a pH measuring instrument, automatic calibration (sloping) is performed by placing the pH electrode in a standard solution, dialing the pH of this standard solution into a decade thumb wheel switch, and depressing the slope switch. The measurement voltage from the electrode is converted to a binary coded digital signal. This signal and the digital signal from the thumb wheel switch are applied to a binary adder which produces a multiple bit output representing the difference. Clock pulses are supplied to a reversible digital counter which counts in a direction specified by the most significant bit of the binary adder output. The count in the reversible counter, representing the error, is converted to an analog signal which is used to control the reference voltage in a ratio system to produce a calibrated reading. The supply of clock pulses to the counter is automatically stopped when the error is zero.

Digital calibration system for an electronic instrument

An automatic calibration system for pH meters includes a digital counter and a digital-to-analog converter which produces an output voltage connected to compensate the zero offset DC voltage of the pH measurement system. During a calibration operation a switch is closed to reset the binary counters and to thereafter supply clock pulses to the counters. As these clock pulses are counted, the digital-to-analog converter produces a staircase output voltage. This is applied to one input of a difference amplifier and the measuring voltage of the pH probe is applied to the other input. A detector stops the application of clock pulses to the counter when the staircase voltage equals the DC offset voltage of the pH measuring probe.

Aage Pettersen

Papers

Digital calibration system for an electronic instrument

Automatic calibration system